1. Field of the Invention
The invention relates to a composite positive electrode active material which, when used in an all solid-state battery; is able to reduce the interfacial resistance with a solid electrolyte material. The invention further relates to an all solid-state battery which uses such a composite positive electrode active material, and to methods of manufacturing the composite positive electrode active material and the all solid-state battery.
2. Description of Related Art
Among the wide variety of batteries that exist, lithium batteries have the special advantage of being lightweight while having a high output and high energy density. For this reason, they are widely used as a power supply for small, portable electronic devices and Personal Digital Assistances (PDAs), and help support today's information-driven society. Lithium batteries are also being considered as a power supply for electric cars and hybrid vehicles. Hence, there exists a desire for even higher energy density, improved safety and larger sizes in lithium batteries.
Because lithium batteries now on the market use organic electrolytes which contain flammable organic solvents, such batteries require safety devices to suppress a rise in temperature at the time of a short circuit and improvements in construction and materials to prevent short circuits. By contrast, all solid-state batteries in which the liquid electrolyte has been replaced with a solid electrolyte layer, making the battery entirely solid state, do not use a flammable organic solvent within the battery. As a result, the safety devices can be simplified, helping to hold down production costs and ensure excellent productivity.
In the field of such all solid-state batteries, efforts are being made to improve the performance of the all solid-state battery by focusing attention on the interface between the positive electrode active material and the solid electrolyte material. For example, a material composed of LiCoO2 (positive electrode active material) which is coated on the surface thereof with LiNbO3 is disclosed in “Narumi Ohta et al. “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries,” Electrochemistry Communications 9 (2007), 1486-1490″. This technology attempts to provide the battery with a higher output by coating LiNbO3 onto the surface of LiCoO2 and thereby lowering the interfacial resistance of LiCoO2 and the solid electrolyte material. Also, WO 2007/004590 discloses a solid-state battery which uses a positive electrode active material that has been surface-coated with a lithium ion-conductive oxide. This attempts to increase the battery output by covering the surface of the positive electrode active material with a lithium ion-conducting oxide and thereby suppressing the formation of a high-resistance layer at the interface between the positive electrode active material and a sulfide solid electrolyte material.
As mentioned in “Narumi Ohta et al, “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries,” Electrochemistry Communications 9 (2007), 1486-1490”, by coating LiNbO3 onto the surface of LiCoO2, it is possible at an initial stage to reduce the interfacial resistance between the positive electrode active material and the solid electrolyte material. However, the interface resistance ultimately rises with the passage of time.
The inventor, upon investigating this, has found that the rise over time in the interfacial resistance is caused by the fact that the LiNbO3 reacts with the positive electrode active material and the solid electrolyte material with which it is in contact, giving rise to reaction products that end up acting as a high-resistance layer. This is believed to be the reason why LiNbO3 has a relatively low electrochemical stability. The inventor then discovered that, when a compound having a polyanion structural moiety with covalent bonds is used instead of LiNbO3, such a compound substantially does not react with the positive electrode active material and the solid electrolyte material.
However, even in cases where a compound having such a polyanion structural moiety is used, if the positive electrode active material contains a transition metal, the interfacial resistance sometimes rises. In WO 2007/004590, a solid battery which uses LiMnO4 as the positive electrode active material and LiTi2(PO4)3 as the lithium ion-conducting oxide is disclosed (see Example 2 of WO 2007/004590). However, because the positive electrode active material includes a transition metal, there is a possibility that the interfacial resistance will rise.